The present invention relates to a drive force transmission apparatus including a torque coupling capable of changing the ratio of torque of a drive source distributed to the main drive wheels and the auxiliary drive wheels of a four-wheel-drive vehicle, a control method of the drive force transmission apparatus, a torque distribution mechanism that distributes torque of a drive source to front wheels and rear wheels, and a limited slip differential having a differential limiting mechanism capable of limiting differential motion between the front and rear wheels.
A four-wheel-drive vehicle having main drive wheels and auxiliary drive wheels are known. The main drive wheels constantly receive torque of a drive source, while the auxiliary drive wheels receive the torque when necessary according to the state of the vehicle. A drive force transmission apparatus is provided between the drive source and the auxiliary drive wheels of the four-wheel-drive vehicle. The drive force transmission apparatus includes a torque coupling capable of changing the torque transmitted to the auxiliary drive wheels (hereinafter, referred to as torque transmission capacity).
When the vehicle turns, the turning radius of the front wheels is greater than the turning radius of the rear wheels. As the turning radius of the vehicle becomes smaller, the difference between the turning radius of the front wheels and the turning radius of the rear wheels, or, the rotation difference between the front wheels and the rear wheels, becomes greater. Accordingly, if the torque transmission capacity is set to a great value when the four-wheel-drive vehicle including the aforementioned drive force transmission apparatus turns at a small turning radius, the rotation difference between the front wheels and the rear wheels cannot be absorbed. In this case, a tight corner braking phenomenon, in which the front and rear wheels act as if they were braked, may occur.
To suppress such disadvantage, a control device described in, for example, Japanese Laid-Open Patent Publication No. 8-2278 may be employed. The control device calculates the turning radius of a vehicle based on the difference between the rotational speed of the outer front wheel and the rotational speed of the inner rear wheel. If the obtained turning radius is greater than or equal to a predetermined reference value, the control device sets the torque transmission capacity to a small value. The reference value is set to the maximum value of the turning radius (hereinafter, referred to as the “maximum radius”) at which the tight corner braking phenomenon may occur. This suppresses the occurrence of the tight corner braking phenomenon.
When the vehicle is in a stopped state, the difference in the rotational speed between the wheels does not occur. Accordingly, a relatively long time elapses from when the vehicle starts to run to when the turning radius of the vehicle can be obtained based on the difference in the rotational speed between the wheels. As a result, for example, immediately after the steering wheel is turned fully to a steering end when the vehicle is in a stopped state and the vehicle starts to run, the fact that the turning radius is greater than the aforementioned reference value may not be detected. In this case, the aforementioned control in which the torque transmission capacity is set to a small value cannot be carried out. This possibly leads to the tight corner braking phenomenon.
The problem is not restricted to the case in which the steering wheel is fully turned when the vehicle is in a stopped state. For example, if the vehicle speed is lower than the upper limit value of the vehicle speed range corresponding to the starting state of the vehicle, the wheel speed sensors may not be capable of accurately detecting the rotational speeds of the corresponding wheels. If, in this case, the steering wheel is steered in such a manner that the turning radius of the vehicle becomes smaller than or equal to the aforementioned maximum radius, the above-described disadvantage can occur.
To suppress the disadvantage, the turning radius of the vehicle may be estimated based on the turning angle of the steering wheel when the vehicle starts. The torque transmission capacity is reduced if the estimated turning radius is smaller than or equal to the maximum radius. However, if the vehicle is started on a road surface with a low friction coefficient such as an icy road surface and the torque transmission capacity is reduced based on the steering wheel turning angle, as has been described, sufficient torque may not be transmitted to the auxiliary drive wheels despite the fact that the vehicle is skidding. This lowers the traction performance of the vehicle.
These problems occur not only with a drive force transmission apparatus having a torque coupling, but also with a torque distribution mechanism that distributes torque of a drive source to front wheels and rear wheels, and with a limited slip differential having a differential limiting mechanism capable of limiting differential motion between a first driveshaft connected to the front wheels and a second driveshaft connected to the rear wheels.